Fix logging/setLevel to return empty hash per MCP specification

This PR fixes `logging/setLevel` to return empty hash per MCP specification.
The MCP specification requires `logging/setLevel` to return an empty result
on success, not nil or the `LoggingMessageNotification` object.

Specification reference:
https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/logging

Fixes #229.

lib/mcp/server.rb

@@ -362,6 +362,8 @@ module MCP
       end
 
       @logging_message_notification = logging_message_notification
+
+      {}
     end
 
     def list_tools(request)

test/mcp/server_test.rb

@@ -810,6 +810,24 @@ module MCP
       assert_instrumentation_data({ method: "resources/templates/list" })
     end
 
+    test "#configure_logging_level returns empty hash on success" do
+      response = @server.handle(
+        {
+          jsonrpc: "2.0",
+          id: 1,
+          method: "logging/setLevel",
+          params: {
+            level: "info",
+          },
+        },
+      )
+
+      assert_equal "2.0", response[:jsonrpc]
+      assert_equal 1, response[:id]
+      assert_empty(response[:result])
+      refute response.key?(:error)
+    end
+
     test "#configure_logging_level returns an error object when invalid log level is provided" do
       server = Server.new(
         tools: [TestTool],